This invention relates generally to techniques of identifying an individual through the identification of the individual's fingerprints, and more particularly to an input sensor unit of fingerprint verification terminal for obtaining an individual's fingerprint using a dry-process or inkless method.
Fingerprint identification systems which actually do not identify the print of a finger but the finger itself when pressed on a contact surface are well known in the art. U.S. Pat. No. 4,053,228 to Michael Schiller discloses a finger identification apparatus of this type which has a transparent glass plate as contact surface for the finger. A source of spatially coherent light provides an interrogating light beam which is directed through the front surface of the glass plate. The light beam is partially reflected at the back surface of the glass plate when a finger is applied to that surface by virtue of optical discontinuities between the glass plate and the finger. There are two different types of discontinuities: one discontinuity takes place between the glass plate and the air underlying the valleys of the finger. Another discontinuity appears between the glass plate and the finger at the crests of the finger. These discontinuities cause differing amounts of light to be reflected; and a reflected signal beam carrying fingerprint information is thus created. This modulated reflected signal beam is correlated against a hologram of the same fingerprint to provide identification.
It has to be understood that the information content of the reflected light beam is highly dependent on the differences of these discontinuities. The reflected light beam may show poor contrasts between crests and valleys of the fingerprint if the finger is not held absolutely immobile while it is in contact with the glass surface. Further blurring or distortions of the fingerprint may result from differing pressure forces during the process of comparison.
Another fingerprint identification system is disclosed in U.S. Pat. No. 4,120,585 to De Palma et al. This invention relates to a pliable optical prism in a fingerprint sensing terminal, the face of which prism is contacted by a finger. Basically the invention comprises an optical element which is to some extend deformable in response to finger pressure, thereby increasing its contact area with a finger. The optical element regains its original shape when the pressure is removed. By proper contact pressure of the finger, a sensing light beam is partially reflected to a photo sensitive device which is activated thereby. This photosensitive device, in turn, activates the sensing terminal. The only reason for the prism to be pliable is to obtain a positive requirement of sufficient finger pressure for activating the system. This known pliable optical prism though having a rippled surface in operation which acts as mirror, still has a very small topographic relief.
The U.S. Pat. No. 3,982,836 issued to Green et al, discloses a method and means for enhancing prints for direct comparison, which method utilizes a transparent pressure-sensitive gel to capture a print pattern. While pressing the finger on the clear plane surface of the transparent pressure-sensitive gel, in the form of a film or tape, contact with the ridges and grooves of the finger causes a relative severe disturbance of the optical quality of the surface. The system includes an elongated pressure-sensitive tape and means for advancing it to expose successive clear portions, so that a number of print images may be made and retained in a single row. An object print formed in the surface of the tape thus constitutes a long-lasting record. A suitable choice of pressure-sensitive material is suggested to obtain a record which may be considered substantially permanent.
A polarized beam of collimated light is passed through the gel. The pattern features impressed in the surface of the gel scatter light out of the optical path. The beam is then directed to a second polarizing element oriented so as to filter out the scattered light rays and thereby to produce a high-contrast print image for direct viewing or further automated processing.
The various fingerprint sensing systems and techniques do not fulfill all practical requirements of a really simple and feasable sensing device of fingerprint verification system which have to be met to provide an apparatus interrogating fingerprints with the highest quality and best contrast for obtaining undistorted information for storing or comparison purposes.
It is therefore an object of the present invention to provide an improved fingerprint verification terminal.
Another object of the invention is to provide an input sensor unit of a fingerprint verification terminal with optical means capable of high resolution of a fingerprint and an improved discrimination of details of the fingerprint independant of differences in individuals reactions to instructions for use or actual appearances of fingers whose prints are to be verified.
A further object of the invention is to provide a finger print verification terminal which sensing results are independent from lasting residues of fingerprints which may for example remain on surfaces of glass sensing plates or glass sensing prisms and may distort further sensing attempts if they are not removed.
Still another object of the present invention is to provide an inkless dry-process fingerprint verification technique which is easily to be used in high performance protection and safeguard systems.